The recovery of oxygen (or of oxygen-enriched gases) from gas mixtures containing oxygen and nitrogen, such as air, is of great interest where industry is concerned, since oxygen finds numerous applications in many fields, especially in the manufacture of steel, of glass or of paper, medicine, the welding of metals, combustion or decontamination.
Many separation processes involve bringing the gas mixture to be separated, containing nitrogen and oxygen into contact, in an adsorption zone, with a zeolitic adsorbent permitting a selective adsorption of nitrogen in the presence of oxygen, so as to recover an oxygen-enriched gas at the exit of the adsorption zone, followed by the regeneration of the zeolite by desorption of the nitrogen. During these various stages the temperature and pressure conditions are set so as to optimize the efficiency of the adsorption and of the desorption of nitrogen. It is well known that low temperatures promote adsorption, whereas elevated temperatures facilitate the nitrogen desorption processes.
Recent studies have shown that the use of lithium-exchanged zeolites, especially lithium-exchanged faujasites, as selective adsorbent for nitrogen, results in a clear improvement in the performance of the PSA or VSA processes for producing oxygen.
However, the high cost of manufacture of such zeolites has hitherto restricted their development. In fact, the lithium salts employed in the manufacture of zeolitic adsorbents are very expensive. The higher the degree of exchange to be reached, the more the quantity of lithium salts needed for carrying out the exchange increases. The exchange of adsorbent particles with lithium cations is therefore unavoidably reflected in an increase in the costs of production of the adsorbent particles and of the oxygen produced.
Thus, when it is desired to employ, in a PSA or VSA process, adsorbents of zeolite type exchanged with lithium to a degree of at least 80%, it is essential to take care not to increase the costs in an unacceptable manner.
A first solution consists in replacing a proportion of the lithium cations with other mono-, di- or trivalent cations. This is proposed in EP-A-0 667 183, which describes an air separation process using a zeolite X exchanged with 50 to 95% of lithium, 4 to 50% of aluminum, cerium or lanthanum and 0 to 15% of other ions. The fact of having to resort to at least two ion exchange stages during the manufacture of the zeolites cannot be considered satisfactory from the industrial and economic points of view. In addition, the performance of such adsorbents has not yet been demonstrated industrially and the ions, in the form of salts, employed for being replaced with lithium ions are also often even dearer than lithium ions.
A second solution consists in replacing a portion of the adsorbent bed of the lithium-exchanged zeolite type with an adsorbent of another type. This solution was adopted in U.S. Pat. No. 5,203,887, which describes an adsorption zone including two adsorbent beds arranged in series; the first consisting of a zeolite X exchanged to at least 80% with lithium, the second consisting of an unexchanged conventional zeolite X. However, the performance obtained by virtue of the use of this additional bed of adsorbent of unexchanged zeolite type is mediocre and much lower, insofar as the process yield and output efficiency are concerned, than those obtained by means of a single bed consisting solely of lithium-exchanged zeolites.
In the prior art there is therefore an existing need for a process, of PSA or VSA type, which would make it possible to obtain yield and output efficiency performances which are at least equivalent, or even superior, to those obtained in the case of sieves comprising only adsorbent particles of zeolite type exchanged with lithium in ratios of at least 80%, or even 90 or 95%, and which would be less of a burden from the viewpoint of costs.